Cleaning device and cleaning process for a plasma reactor

ABSTRACT

The invention concerns a device and a process, the device being a cleaning device utilizing a dry chemical means assisted by plasma from a reactor ( 10 ) containing an unwanted deposit on its walls and at least one other polarizable surface ( 12 ), characterized in that it comprises means ( 13, 14 ) for positively polarizing one or each of the polarizable surfaces relative to the reactor walls maintained at a reference potential.

FIELD OF THE INVENTION

The invention concerns a device for cleaning a reactor.

The invention concerns more precisely a device for plasma-assisted drychemical cleaning of a reactor having an undesirable deposit.

The invention concerns the method of cleaning a reactor implemented withthe device according to the invention.

In general terms, the invention applies to reactors using dry methodsand the walls of which are covered with undesirable deposits.

The invention applies to the cleaning of deposition and surfacetreatment reactors.

This is the case, for example, with PACVD (plasma-assisted chemicalvapour deposition) and PAPVD (plasma-assisted physical vapourdeposition) reactors or etching reactors, in methods acting on theetching-deposition balance (selective SiO₂ etching with respect tosilicon, anisotropic etching by lateral passivation).

The invention can also apply to CVD (silicon, tungsten), laserdeposition or MBE (molecular beam epitaxy) reactors, the walls of whichit is necessary to clean periodically to prevent redeposition or dust onthe surfaces.

PRIOR ART

The problem of the cleaning of reactors in general, and depositionreactors in particular, assumes crucial importance in electronics,optics and micro-nanotechnology, but also in surface treatments (metalspraying, or deposition of hard, tribological or anticorrosion layers)in very many industrial sectors. The cleaning time and therefore thetime for immobilisation of the production reactors represents asignificant cost. Despite this cost, the cleaning of the reactors is anecessity for preserving quality and reproducibility for the processesimplemented.

For this reason, in microelectronics, the trend is for periodic cleaning(every “n” deposition, or even after each deposition), usually bychemical cleaning by liquid or dry method (e.g. plasma). This is thecase for example with the cleaning of reactors depositing Si, SiO₂ orSi₃N₄ in reactors of the capacitive RF type (radio frequency dischargebetween two parallel electrodes). In this case, the equipmentmanufacturers have different strategies such as:

i) cleaning of the reactors in a liquid bath (wet chemical method), butthe time taken for dismantling and treating the reactor is prohibitiveand requires two sets of treatment parts;

ii) plasma etching using gentle processes, that is to say by purelyreactive dry chemical method.

For this reason, the majority of equipment manufacturers have developedreactor cleaning methods using auxiliary gas plasmas that are asreactive as possible vis-{dot over (a)}-vis the deposits to be removedby reactive chemical method, and at relatively high pressure in order toproduce the greatest possible concentrations of reactive species (forexample atomic fluorine F for chemical etching of Si, SiO₂, Si₃N₄ or W,atomic oxygen O for the etching of carbon or polymers) and heating thewalls in order to thermally activate the chemical etching reactions.

Thus it is possible to clean SiO₂ or Si₃N₄ on walls of reactors coveredwith these deposits by raising them to 300° to 400° C. in the presenceof an NF₃ plasma generated by inductive RF coupling or by surface wave).However, in general, this type of cleaning is possible only if stablevolatile reaction products can be formed from the various elements ofthe deposit to be removed.

The dry cleaning techniques proposed above often have either often highcleaning times by purely chemical method, or high chemicalaggressiveness on certain components of the reactor at high temperature(fluorine with nickel, oxygen with the dielectric insulators), and hencehigh cleaning cost.

They usually require the use of an auxiliary plasma source operating ata higher pressure than the plasma used for the process, and the choiceof exotic gases or gases difficult to use, which also poses the problemof the cost of the gases (high flow rates), the retreatment of high gasvolumes discharged from the reactor, and safety through the manipulationof dangerous gases (toxic, corrosive, flammable or explosive). Anexample of this is given in the document US 2005/0224458.

The investment and operating costs incurred by current techniques aretherefore extremely high.

One objective of the invention is therefore to propose a cleaning devicerequiring only minor modifications to the reactor.

An objective of the invention is to propose a simplified cleaning methodrelying on plasma-assisted dry chemical etching methods, in particularusing ion bombardment.

It is this ion bombardment that it is proposed to control in theinvention.

SUMMARY OF THE INVENTION

To achieve these objectives, there is provided in the context of thepresent invention a method for the plasma-assisted dry chemical cleaningof a reactor having an undesirable deposit on its walls and on at leastone other biasable surface, characterised in that at least one sequence,referred to as a positive sequence, of cleaning the walls of the reactorwas implemented by positive biasing of the or each biasable surface,with respect to the walls of the reactor, the walls being at areferenced potential.

The method according to the invention can also have at least one of thefollowing characteristics:

-   -   at least one other sequence is implemented, referred to as a        negative sequence, for cleaning the or each biasable surface by        negative biasing thereof, with respect to the walls of the        reactor;    -   a negative sequence followed by a positive sequence is        implemented;    -   at least one alternating succession of a positive sequence and a        negative sequence is implemented;    -   a plurality of periodic alternating successions of a positive        sequence and a negative sequence are implemented;    -   a biasing to referenced voltage of the or each biasable surface        is implemented during the positive sequence;    -   a biasing to referenced voltage of the or each biasable surface        is implemented during the negative sequence;    -   an auto-biasing of the or each biasable surface is implemented        during the negative sequence.

To achieve these objectives, there is also provided in the context ofthe present invention a device for the plasma-assisted dry chemicalcleaning of a reactor having an undesirable deposit on its walls and onat least one other biasable surface, characterised in that it comprisesmeans for positively biasing, with respect to the walls of the reactormaintained at a referenced potential, the or each biasable surface.

The device according to the invention can also have at least one of thefollowing characteristics:

-   -   it comprises means for negatively biasing, with respect to the        walls of the reactor, the or each biasable surface;    -   it comprises means for successively alternating the sign of the        biasing, with respect to the walls of the reactor, of the or        each biasable surface;    -   it comprises a DC voltage generator; electrically connected to        the or each biasable surface, and means of controlling the        generator for successively delivering positive and negative        voltages, with reference to the walls of the reactor;    -   it comprises means for successively and periodically alternating        the sign of the biasing, with respect to the walls of the        reactor, of the or each biasable surface;    -   it comprise a periodic voltage generator, connected electrically        to the or each biasable surface by means of a circuit comprising        a low-impedance capacitor and means for short-circuiting the        capacitor;    -   it comprises a periodic voltage generator, in direct electrical        connection with the or each biasable surface and referenced with        respect to the walls of the reactor, which are preferably        earthed.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics, aims, advantages and objectives of the presentinvention will emerge from a reading of the following detaileddescription, and with regard to the accompanying drawings, given by wayof non-limitative examples and on which:

FIG. 1 depicts by way of example DC voltages applied to at least onebiasable surface of a reactor, according to the two types of sequence,with respect to the referenced wall potential V_(w);

FIG. 2 depicts, by way of example, a periodic voltage signal V(t)supplied to at least one biasable surface, during a sequence referred toas sequence 1 or negative sequence, in which the biasable surfaceauto-biases negatively with respect to the referenced potential of thewalls V_(w);

FIGS. 3 a and 3 b depict, by way of examples, periodic voltage signalssupplied and applied to at least one biasable surface, during a sequencereferred to as sequence 2 or positive sequence, in which the biasablesurface periodically has a positive potential with respect to thereferenced potential of the walls V_(w);

FIG. 4 depicts, by way of example, a period voltage signal supplied andapplied to at least one biasable surface, in which the biasable surfaceperiodically has a positive and negative potential with respect to thereferenced potential of the walls V_(w);

FIGS. 5, 6, 7 and 8 depict schematically different variant embodimentsof a device according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The method according to the invention is a plasma-assisted chemicaletching cleaning method.

This can use plasma etching methods conventional in microelectronics andmicrotechnologies, such as for example:

i) etching of carbon C or carbon compounds (DLC, polymers, etc) byplasmas containing oxygen,

ii) etching of silicon Si or tungsten W by plasmas based on fluorine(SF₆, CF₄, NF₃, etc),

iii) etching of SiO₂ or Si₃N₄ by plasmas based on fluorine (SF₆, CF₄,NF₃, etc),

iv) etching of Al, B or other compounds by plasmas based on bromine orchorine (Br₂, Cl₂, HBr, etc).

So that the cleaning method according to the invention can beimplemented, it is necessary for the plasma to be produced in a reactorwith walls at referenced potentials (for example earthed).

The cleaning operation comprises at least one cleaning sequence (byplasma etching) in which the walls of the reactor are cleaned bypositive biasing, with respect to the walls of the reactor, of the oreach biasable surface (for example substrate carrier or electrode). Thissequence or sequences are called sequence 2 or positive sequence in theremainder of the description.

The operation comprises, according to the application, at least oneother sequence of cleaning the or each biasable surface (for examplesubstrate carrier and/or electrode) by negative biasing of the biasablesurface with respect to the walls of the reactor. This sequence orsequences are called sequence 1 or negative sequence in the remainder ofthe description.

Cleaning of a Conductive Deposit

If the deposit to be cleaned is conductive (C, W, Si etc) and thebiasable surface does not need to be cleaned (because of its dispositionwithin the reactor for example, or because it carries a substrate duringthe deposition operations, or because the deposit is considered to benegligible), a sequence 2 may suffice: there is then complete cleaningof the walls of the reactor (chamber) by application to the biasablesurface (substrate carrier or electrode) of a positive DC voltage withrespect to the walls.

If the deposit to be cleaned is conductive (C, W, Si etc) and thebiasable surface is also to be cleaned, two successive sequences onlymay suffice, namely a sequence 1 and a sequence 2: there is thencomplete cleaning of the biasable surface by application to this surface(substrate carrier or electrode) of a negative DC voltage with respectto the walls (for example earthed) and the complete cleaning of thewalls of the chamber by application to the surface (substrate carrier orelectrode) of a positive DC voltage with respect to the walls.

Reference can be made to FIG. 1, where the voltage V(t) supplied to thebiasable surface and to which it is raised changes from a sequence 1,referenced 1, to a sequence 2, referenced 2, and where V_(w) is thepotential of the walls of the reactor.

Note that, in the context of the invention, it is of little importancewhether sequence 1 is carried out followed by sequence 2 or vice versa.However, in practice, it is more suitable to commence with sequence 1,that is to say to carry out the cleaning of the biasable surface.

The biasable surface (substrate carrier or electrode) can be smallcompared with the surface of the walls to be cleaned but must have asufficiently large size to disturb the plasma, so that the potential ofthe plasma V_(p) is positioned, at each moment t, at a value that isalways more positive than the most positive surface (wall or electrode).

The condition to be fulfilled for constituting a substrate carrier or alarge-size electrode is that the ratio of the surface S_(s) of thesubstrate carrier or large-size electrode to the surface S_(w) of thewalls is at least greater than approximately 1.5 times the square rootof the ratio of the mass m_(e) of the electrons to of the ions m_(i),that is to say:

S _(s) S _(w)>1.5(m _(e) /m _(i))^(1/2)   (1)

For argon, it is therefore necessary for the ratio to be greater than1/180 ( 1/30 for hydrogen and 1/500 for xenon), which is generally thecase with substrate carriers or electrodes used in microelectronics orin surface treatments by furnace.

Below the ratio given by equation (1), the surface S_(s) does notdisturb the plasma, which corresponds to the case of electrostaticprobes or Langmuir probes. In this case, the plasma potential V_(p)remains practically unchanged compared with the potential of the walls(generally earthed) if the biasable surface of the surface S_(s) ispositively biased with respect to the potential V_(w) of the walls. Theion bombardment energy W_(w) of the walls of the reactor is then closeto:

W _(w) ≈e(V _(p) −V _(f))=(kT _(o)/2) [1+In (m _(i)/2πm _(e))]   (2)

where k is Boltzmann's constant, e the charge on the electron, T_(e) isthe electron temperature and V_(f) the potential of the biasable surfacewhich in floating, then equal as a first approximation to the potentialof the walls (V_(f)≈V_(w)).

On the other hand, in the general case that interests us here, that of aratio S_(s)/S_(w) higher than the value given by equation (1), thebiasable surface S_(s) profoundly disturbs the equilibrium of the plasmaif it is raised to a positive potential V₀ with respect to the potentialof the walls V_(w) (sequence 2). In this case, the plasma potentialV_(p) is offset by the value V₀−V_(w) and the ion bombardment energyV_(w) of the walls is, as a first approximation, for a DC voltage V₀,equal to:

W _(w) ≈e (V _(p) −V _(w))=e(V _(p) −V _(f) +V ₀ −V _(w))   (3)

In other words, by positively biasing the biasable surface S_(s) to theDC potential V₀ with respect to the potential of the walls V_(w)(sequence 2), it is possible to adjust to the required value the ionbombardment energy on the walls during sequence 2. During this sequence,the ion bombardment energy W_(s) of the electrode is equal to:

W _(s) ≈e(V _(p) −V _(f))   (4)

If now the surface S_(s) is negatively biased to the DC potential −V₀with respect to the potential of the walls V_(w) (sequence 1), the ionbombardment energy W_(s) of the surface S_(s) is equal to:

W _(ε) =e(V _(p) −V _(s))=e(V _(p) −V _(f) +V _(w) −V ₀)   (5)

while the ion bombardment energy W_(w) of the walls during this samesequence is equal to:

W _(w) =e(V _(p) −V _(f))   (6)

It can be seen therefore that the bombardment energies of the walls andlarge-size electrode are reversed when changing from sequence 1 tosequence 2.

In the case of the cleaning of a conductive deposit, on the walls of thereactor and on at least one other biasable surface of the reactor, it istherefore possible to clean the reactor in two stages:

-   -   cleaning of the large-sized biasable surface (substrate carrier        or electrode) during sequence 1 by adjusting the energy of the        ion bombardment W_(s) by means of the value −V₀ of the DC        voltage applied to the large-sized biasable surface (FIG. 1),        and    -   cleaning of the walls of the reactor during sequence 2 by        adjusting the energy of the ion bombardment W_(w) by means of        the value +V₀ at the DC voltage applied to the large-sized        electrode (FIG. 1).

This is because, since the rate of chemical etching caused by the ionbombardment increases rapidly with the energy, a method of rapidcleaning of the reactor requires a more energetic ion bombardment thanthat due solely to the difference between plasma potential and floatingpotential of the biasable surface.

In equations (3) and (5) and in FIG. 1, +V₀ and −V₀ are opposite values,but nothing makes it necessary to take symmetrical biasing values V₀.However, as it is preferable to remain below or close to the values ofthe sputtering thresholds of the reactor, it is more convenient tochoose symmetrical values as in the previous example, with typicallyV₀−30 to 100 V.

As mentioned above, a sequence 2 or a succession of a sequence 1 and asequence 2 may suffice according to circumstances. However, if thecleaning of the walls contaminates the large-sized biasable surface, orvice versa, it is preferable to provide a certain number of alternatingsequences until there is complete cleaning of the reactor (walls andlarge-sized biasable surface).

It should be noted that, in the case where the substrate carrier is usedas a large-sized electrode, some parts concealed by the substrate mayprove to be free of deposit. In fact the deposit may affect solely theedges of the substrate carrier but in this case it is preferable toclean the whole of the substrate carrier (sequence 1).

Cleaning of an Electrically Insulating Deposit

If the deposit to be cleaned is insulating (SiO₂, Si₃N₄, etc) and thebiasable surface does not need to be cleaned (no insulating deposit), asequence 2 may suffice.

In the general case, it is however essential to apply a succession ofperiodic alternating sequences comprising the two sequences describedpreviously:

-   -   partial cleaning of the biasable surface (substrate carrier or        electrode) during the negative half wave with respect to the        walls (generally earthed) of a periodic voltage applied to the        biasable surface, and    -   partial cleaning of the walls during the positive half wave        (with respect to the walls) of a periodic voltage applied to the        biasable surface.

These sequences are applied until there is complete cleaning of thereactor.

Here also, it should be noted that, in the context of the invention, itis of little importance whether sequence 1 is carried out followed bysequence 2 or vice versa. However, in practice, it is more suitable tocommence with sequence 1, that is to say to carry out the cleaning ofthe biasable surface.

Here also, the ratio of the surface areas of the biasable surface andthe walls of the reactor must comply with equation (1).

The periodic voltage of frequency f₀, supplied for example by a periodicvoltage generator, is referenced with respect to the potential of thewalls of the reactor (generally earthed), which is not in principle thecase with the biasable surface (auto-biasing mode is thought of).

This is because, in the case of the cleaning of an insulating deposit,it is no longer possible to clean the reactor with alternating DCbiasing voltages (FIG. 1), and it is therefore necessary to haverecourse to periodic biasing to a higher frequency by capacitive effect.

Thus, during a first sequence (sequence 1 or negative sequence) of theperiodic biasing, it is usual to carry out negative auto-biasing of thebiasable surface (substrate carrier or large-sized electrode) byapplying a periodic voltage to the biasable surface through alow-impedance capacitor (the current case of RF auto-biasing) andtherefore to clean the large-sized biasable surface by chemical etchingassisted by ion bombardment (plasma).

During this sequence 1, an adapted means therefore supplies a periodicvoltage signal V(t) (such as the one illustrated in FIG. 2) to thebiasable surface which, for its part, biases itself (auto-biasing) sothat it receives, during a period, as many positive charges (ions) asnegative charges (electrons) having regard to the voltage values takenby this signal and that of the plasma potential.

This auto-biasing results from the non-linearity of the current/voltagecharacteristic of the plasma.

Once the biasable surface (substrate carrier or electrode) is clean, theinvention consists of applying to it, during a second sequence (sequence2 or positive sequence), a periodic voltage referenced (for example tothe potential of the walls, generally earthed) so that the voltageapplied to the biasable surface during the positive half wave takes apositive value +V₀ with respect to the potential of the walls.

Examples of periodic voltage signals supplied by a means adapted to thebiasable surface are illustrated on FIG. 3 a and FIG. 3 b. On FIG. 3 a,the signal supplied by the adapted means is that carried by the biasablesurface. On the other hand, on FIG. 3 b, the signal supplied by theadapted means is carried to the biasable surface solely when V>V_(w),(sequence 2), the biasable surface therefore “sees” the same signal asat FIG. 3 a.

In other words, it is thus possible to clean the biasable surface duringsequence 1 by plasma assisted chemical etching and to clean the walls ofthe reactor in the same way during sequence 2.

According to a variant of the invention, a periodic voltage referenced(for example to the potential of the walls, generally earthed) isapplied to the biasable surface so that the voltage applied to thebiasable surface during the negative half waves (sequences 1) takes anegative value V₀ with respect to the potential of the walls, and sothat the voltage applied during the positive half waves (sequences 2)takes a positive value +V₀ with respect to the potential of the walls(FIG. 4).

In this variant, it will be understood that there is no longer anynegative auto-biasing of the biasable surface (except as long as thereremains an insulating deposit). To do this, the low-impedance capacitoris short-circuited (FIG. 6) so that the voltage supplied by adaptedmeans corresponds to the voltage carried to the biasable surface.

Compared with the case of cleaning by successive applications of DCvoltages, the auto-biasing values calculated in the case of theapplication of periodic voltages are more complex than in equations (2)to (6) presented above and depend more or less greatly on the angularvelocity ω₀=2π f₀ (with respect to the ion plasma angular velocity), theshape of the periodic signal or the ratio of the surfaces S_(s) andS_(w).

It should be noted however that the angular velocity ω₀ of the cleaningsequences is not a limitative given of the method. In particular, thisangular velocity may be smaller or greater than the ion plasma angularvelocity ω_(pi) defined by ω_(pi) ²−n e²/ε₀m_(i) where n is the densityof the plasma, −e the charge on the electron, m_(i) the mass of the ionsof the plasma and ε₀ the permittivity of the vacuum.

It should also be noted that the shape of the periodic signal (thesuccession of periodic alternating sequences) may be sinusoidal,rectangular or other.

However, the values calculated in equations (2) to (6) are in practicevalid in the case of the application of rectangular signals or periodicangular velocity voltages ω₀<<ω_(pi) and highly asymmetric surfacesS_(s) and S_(w).

It should also be noted that the method of cleaning by the applicationof periodic voltages as described above make sense only if the ratio ofthe surfaces S_(s) and S_(w) is small (S_(s)/S_(w)<<1). This is because,if the surfaces S_(s) and S_(w) are close dimensions (S_(s)≈S_(w)), asingle auto-biasing sequence suffices since the biasing of the twosurfaces is then symmetrical with respect to the signal applied, eachsurface being in turn biased negatively with respect to the plasmapotential.

The cleaning by the succession of periodic alternating sequencespresented above when the deposit to be cleaned is insulating may also beapplied to the cleaning of conductive deposits. However, the cleaning ofconductive deposits in two or more sequences under DC voltage procuresthe advantage of not requiring a periodic voltage generator, much moreexpensive that a DC supply.

As mentioned above and as far as possible, the voltages applied (whetherthe deposit be conductive or insulating) must be adjusted so that theion bombardment energy of the electrode or walls of the reactor remainsbelow or close to the sputtering threshold of the materials that makethem up (negligible sputtering of the electrode and walls of thereactor) that is to say typically below 100 eV.

The method according to the invention described above is implementedwith means detailed below.

More precisely, the cleaning devices according to the inventioncomprise, and this in a manner known in the plasma assisted dry chemicalcleaning devices, means of producing, in a reactor, a reactive plasmacapable of forming volatile reaction products with the deposits to beremoved.

These means also comprise a biasable surface (substrate carrier orelectrode) of surface area S_(s) sufficient with respect to the surfacearea S_(w) of the walls [equation (1)] to allow modification of theplasma potential. The biasable surface is then said to be of large size.

These means also comprise means of applying to the large-size biasablesurface excessive biasing sequences using DC or periodic voltagesaccording to the method of the invention.

Different devices according to the invention can be described. Inparticular, the plasma production means can consist of different typesof plasma such as microwave plasmas for example distributed electroncyclotron resonance (DECR) plasmas, multi-dipole plasmas (non-limitativeexamples), such as continuous or radio-frequency capacitive discharges,diodes or triodes, where one of the electrodes is used as a biasablesubstrate carrier, or such as inductive discharges with inductorinternal to the chamber. In all these examples, the substrate holdersgenerally comply with the criterion of surface area ratio of equation(1).

With regard to the application of successive sequences of DC voltages(solely for conductive deposits), a controlled supply capable ofsuccessively delivering positive and negative continuous voltages, isnecessary.

For application of a periodic negative auto-biasing voltage (sequence 1,insulating deposit), a conventional means consists of a generatorcapable of delivering a periodic voltage through a low-impedancecapacitor.

On the other hand, obtaining a positive biasing voltage during thepositive half wave (sequence 2, insulating deposit) requires a generatorcapable of delivering a periodic voltage referenced with respect to adefined potential.

In other words, the invention requires, in very many cases, a generatorcapable of successively delivering either a periodic voltage through alow-impedance capacitor (biasing of the substrate during the methodpreceding the cleaning and/or during sequence 1), or a referencedperiodic voltage having during the positive half wave (sequence 2) apositive voltage with respect to the potential of the wall.

According to the variant embodiment mentioned above in the context bfthe method (sequence 1, insulating deposit), a device according to theinvention will then have to require a generator capable of delivering aperiodic voltage referenced with respect to the wall, that is to sayhaving a periodically negative and then positive voltage, symmetricalfor example with respect to the potential of the wall.

Thus a first device comprises means of applying DC voltages that arenegative and positive, with reference to the walls of the reactor, to abiasable surface (a substrate carrier for example), and thisindependently or not of the production of the plasma.

This first device is illustrated in FIG. 5. It has a reactor 10referenced to earth 11, a biasable surface 12 electrically connected bya means 13 to a DC voltage generator 14, comprising means of controllingthe generator in order to successively deliver positive and negativevoltages, with reference to the walls of the reactor.

A second device (FIG. 6) comprises a periodic voltage generator 140,electrically connected to the or each biasable surface 12 by means of acircuit 15 comprising a low-impedance capacitor 151 and means 152 forshort-circuiting the capacitor. With this device, the biasing of thebiasable surface takes place independently or not of the production ofthe plasma.

The short-circuiting means 152 may for example, but non-limitatively, beformed by a switch, disposed in parallel to the low-impedance capacitor.In this case, the switch in open during a sequence 1 (FIG. 6 and FIG.2), so that the voltage generated by the generator passes through thecapacitor before arriving at the or each biasable surface. On the otherhand, the switch is closed for a sequence 2, so that the voltagegenerated by the generator is the voltage of the or each biasablesurface (FIG. 3 a and FIG. 3 b).

A third device comprises means of applying a periodic voltage, forexample by a periodic voltage generator 141, to the or each biasablesurface 12, by means of a direct electrical connection 13 (FIG. 7). Thisvoltage is referenced to the potential of the walls of the reactor(generally earthed), and this independently or not of the production ofthe plasma.

It should be noted that, in the case of capacitive discharges betweenelectrodes, the biasing is not independent of the production of theplasma, whereas it is generally so in inductive plasmas and microwaveplasmas.

One of the main advantages of the invention is its simplicity in termsof the method and device compared with current techniques.

This is because the sequence 1 or negative sequence mentioned in thedescription above uses known plasma-assisted etching methods in so faras the or each biasable surface is negatively biased with respect to theplasma potential (which is slightly greater than the potential of thewalls).

The sequence 2 or positive sequence, for its part, therefore modifiesthe nominal functioning of the reactor in order to achieve an objectiveof cleaning the walls of the reactor, which are for this purpose put toa referenced potential.

For certain types of applications known to persons skilled in the art,use is made, in addition to a cathode (which constitutes a biasablesurface) negatively biased with respect to the plasma potential, atleast one anode intended to collect the electrons, which is, in nominalfunctioning, biased positively with respect to the plasma potential.

However, in this type of application and in order to avoid insulatingdeposits on this anode or these anodes, means are provided fornegatively biasing the anode or anodes with respect to the plasmapotential (the walls are not referenced in this type of application).This method, which can be likened to a sequence 1 as described abovefrom the point of view of the anode or anodes, is implemented for thepurpose of ensuring continuity of nominal functioning of the reactor, byensuring that the anode fulfils its role of electron collector. This isthe case for example with the document EP 1 458 006.

Among the advantages that the invention procures, it can in particularbe mentioned that it:

1) implements conventional etching methods, tried and tested and wellmastered;

2) does not require any modification to the reactor architecture;

3) requires only minor modifications to the biasing supplies for thebiasable surface (substrate carrier or electrode);

4) does not cause any sputtering of the biasable surface if the biasingthereof is independent of the production of the plasma (for exampleplasmas produced by microwaves), and is maintained at a value less thanthat corresponding to the sputtering of the surfaces of the reactor;

5) offers a method that is scarcely damaging for the reactor or theenvironment.

To give a clear idea, the applicant supplies below an example of atypical application of its invention, given by way of non-limitativeexample.

A capacitive discharge is considered in a reactor 10 connected to earth11 created between a biasable surface (a substrate carrier) to which avoltage is applied and an earthed electrode 15 (FIG. 8) or also areactor of the same dimensions in which the plasma is produced bymicrowaves and where the same substrate carrier can be biased by avoltage. The element 16 corresponds, according to the type of voltagegenerated, to the generator 14 in FIG. 5, to the assembly formed by thegenerator 140 and the circuit 15 in FIG. 6, or to the generator 141 inFIG. 7.

The ratios of the earthed surfaces and those biasable continuously orperiodically (RF) is 700 cm²/7000 cm², that is to say 1/10, whichperfectly corresponds to the criterion defined by equation (1) and tothe case of a substrate-carrier surface that is small compared with thesurface of the walls.

A well known example is the removal of the deposits of SiO₂ on the wallsof a deposition reactor. In the context of the invention, this depositcan be removed by means of an SF₆ plasma by the formation of thereaction products SiF₄ and O₂ by chemical etching assisted by ionbombardment. It should be noted that the use of CF₄, and more generallyfluorocarbon gases, may, according to the plasma parameters, lead to aCF_(x) deposit of the Teflon type.

1. Method of cleaning, by plasma-assisted dry chemical method, a reactor(10) having an undesirable deposit on its walls and on at least oneother biasable surface (12), characterised in that there are effected aplurality of alternating successions of: a so-called negative sequence(1) of cleaning the or each biasable surface by negative biasingthereof, with respect to the walls of the reactor; and a so-calledpositive sequence (2) of cleaning the walls of the reactor by positivebiasing of the or each biasable surface, with respect to the walls ofthe reactor, the walls being at a referenced potential.
 2. Methodaccording to the preceding claim, characterised in that a plurality ofperiodic alternating successions of a negative sequence (1) and apositive sequence are implemented.
 3. Method according to one of thepreceding claims, characterised in that a biasing to a referencedvoltage of the or each biasable surface (12) is performed during thepositive sequence (2).
 4. Method according to one of the precedingclaims, characterised in that a biasing to a referenced voltage b of theor each biasable surface (12) during the negative sequence (1) iscarried out.
 5. Method according to one of the preceding claims,characterised in that an auto-biasing of the or each biasable surface(12) is carried during the negative sequence (1).
 6. Device forcleaning, by plasma-assisted dry chemical method, a reactor (10) havingan undesirable deposit on its walls and on at least one other biasablesurface (12), characterised in that it comprises: means (13, 14, 140,15, 16) for negatively biasing, with respect to the walls of thereactor, the or each biasable surface; means (13, 14, 140, 15, 16) forpositively biasing, with respect to the walls of the reactor maintainedat a reference potential, the or each biasable surface; means (14, 15,140, 16) for successively alternating the sign of the biasing, withrespect to the walls of the reactor, of the or each biasable surface. 7.Device according to the preceding claim, characterised in that itcomprises a DC voltage generator (14) electrically connected to the oreach biasable surface, and means of controlling the generator in orderto successively deliver positive and negative voltages, with referenceto the walls of the reactor.
 8. Device according to one of claim 6 or 7,characterised in that it comprises means for successively andperiodically alternating the sign of the biasing, with respect to thewalls of the reactor, of the or each biasable surface.
 9. Deviceaccording to the preceding claim, characterised in that it comprises aperiodic voltage generator (140), electrically connected to the or eachbiasable surface by means of a circuit (15) comprising a low-impedancecapacitor (151) and means (152) for short-circuiting the capacitor. 10.Device according to claim 7, characterised in that it comprises aperiodic voltage generator (140) in direct electrical connection (13)with the or each biasable surface and referenced with respect to thewalls of the reactor, which are preferably earthed.
 11. Device accordingto one of claims 7 to 10, characterised in that the biasable surface hasa dimension such that it satisfies the inequality:S _(s) /S _(w)>1.5(m _(e) /m _(i))^(1/2) so as to disturb the plasma, sothat the potential of the plasma V_(p) is positioned, at each moment t,at a value more positive than the most positive surface of the reactor,and where S_(s) is the biasable surface, S_(w) the surface of the wallsof the reactor, m_(e) the mass of the electrons of the plasma and m_(i)that of the ions of the plasma.